The way i understand it is that the harness points are mounting points the more points you have the mor positions its located in if youlook at 3 points the shoulder straps are one lonh length the for a "V" shape that go up one side loop round an eyelet and com back down on side , on 4 points these are 2 seperate lengths that run parallell to each other,and to 2 seperate mountings at the rear , as for either one being safer than the other im not sure 4 points are a little more supportive because you can adjust most models slighly more than the 3 point , as for being pinned in a car with ya head squised this would happen with a normal "inertia type belt , just as it would with a harness (i know from personal experiance)